llvmPackages_{13,git}.clang: build clang-tools-extra

This is already done for previous versions of clang which use
a release tarball, but must be done differently for the more
recent versions which use fetchFromGitHub.

Fixes clang-tools clangd wrapper

pkgs/development/compilers/llvm/13/clang/default.nix

@@ -44,6 +44,8 @@ let
     ];
 
     postPatch = ''
+      (cd tools && ln -s ../../clang-tools-extra extra)
+
       sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
              -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
              lib/Driver/ToolChains/*.cpp

pkgs/development/compilers/llvm/git/clang/default.nix

@@ -44,6 +44,8 @@ let
     ];
 
     postPatch = ''
+      (cd tools && ln -s ../../clang-tools-extra extra)
+
       sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
              -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
              lib/Driver/ToolChains/*.cpp